In view.php the drop down field for tag selection shows only 'Existing Tags'.

When I look into the source code I find:

<option value="0">Existing tags</option>
<p style="color:red">SYSTEM WARNING: 'Invalid argument supplied for foreach()' in 'C:\[...]mantis\core\print_api.php' line 321</p>

So here the problem seems to be with

  $query = "SELECT t.id FROM $t_tag_table t
  LEFT JOIN $t_bug_tag_table b ON t.id=b.tag_id
  WHERE b.bug_id IS NULL OR b.bug_id != " . db_param();

in tag_api.php line 468.

As soon as I change this SQL, like

  $query = "SELECT t.id FROM $t_tag_table t
  LEFT JOIN $t_bug_tag_table b ON t.id=b.tag_id
  WHERE b.bug_id IS NULL ";

or

  $query = "SELECT t.id FROM $t_tag_table t
  LEFT JOIN $t_bug_tag_table b ON t.id=b.tag_id
  WHERE b.bug_id IS NULL OR b.bug_id != 1";

I receive the error:
Database query failed. Error received from database was 0002812: Could not find stored procedure 'NE'. for the query: SELECT t.id FROM mantis_tag_table t
LEFT JOIN mantis_bug_tag_table b ON t.id=b.tag_id
WHERE b.bug_id IS NULL OR b.bug_id != 1.

When I use the second query ( OR b.bug_id != 1 ) directly in the database I get a result.

db_param() returns '?'. When I run the query "[...]OR b.bug_id != ?" directly in the database I also get an error.

My guess is a problem with the SQL code and MS SQL but I was not able to solve it.